1. Field of the Invention
The present invention relates to a control device, a control method, and a storage apparatus which read and write data by causing a head to fly over a rotating recording medium and particularly relates to a storage apparatus, a control method, and a control circuit which control a clearance (spacing) between the head and the recording medium surface by thermal expansion caused by power distribution to and heating of a heater provided in the head.
2. Description of the Related Arts
Conventionally, the clearance between a head and a recording surface of a magnetic disk has to be reduced in order to realize a high recording density of a magnetic disk apparatus, and it is designed so that the clearance caused by rise of the head becomes constant by utilizing the pressure (positive pressure/negative pressure) of air that flows into the space between a slider, in which the magnetic head is disposed, and a magnetic recording medium. In order to improve the recording density, the distance between the head and the medium has to be reduced; therefore, the clearance of the head has been reduced year by year along with increase in the density of the recording density, and, recently, the clearance of 10 nm order has been realized. Since the conventional head maintains the clearance merely by the pressure of the air, change in the clearance due to the environmental temperature, mounting variation caused when the magnetic head is mounted, and variation in the clearance caused in each head like production variation caused in each head cannot be corrected. Therefore, the clearance has to be designed under the conditions, in which the variations are taken into consideration; however, since variation in the clearance of each head is present within a mechanical tolerance range, when medium contact is taken into consideration, there is a problem that the clearance cannot be set smaller than the tolerance range. Moreover, since the medium rotates at a high speed in the magnetic disk apparatus, when the clearance of the head is reduced, collision with minute protrusions on the magnetic disk surface readily occurs; and, when the medium and the magnetic head are brought into contact with each other, there is a problem that the life of the apparatus cannot be satisfied because of wear of the head, contamination (dust), etc. generated when the head and the medium are brought into contact with each other. Therefore, recently, like Patent JP2002-037480 (Japanese Republished Patent Publication), a method in which change in the protrusion value (TPR value) due to phenomenon in which the head flying surface protrudes toward the direction of the magnetic disk due to temperature increase of the write head (Thermal Protrusion: TPR) is measured and retained on the magnetic disk in an examination step or the like, and the clearance is controlled for each head by using the data has been proposed. Furthermore, methods like Patent JP2005-071546 and JP2005-276284 in which a heater is incorporated in the head, and the clearance between the head and the recording surface of the magnetic disk is controlled by utilizing the protrusion phenomenon which is caused by thermal expansion of the head flying surface along with power distribution of the heater have been proposed. In JP2005-071546 2, a constant element temperature is maintained by changing the electric power applied to an electrically conductive film provided in the head with respect to increase in the element temperature which is due to the apparatus temperature or recording/reproduction, so that a constant clearance is maintained between the element and the recording medium. In JP2005-276284, a clearance-increasing heating device which increases the distance between a recording/reproduction element and the magnetic disk surface by causing a part of an air bearing surface of the head to expand and protrude by heating and a clearance-reducing heating device which reduces the distance between the recording/reproduction element and the magnetic disk surface by causing another part of the air bearing surface of the head to expand and protrude by heating are provided in the head, and the clearance is corrected so that reproduction can be carried out without causing collision upon, for example, activation of the apparatus.
However, in such conventional methods that control the clearance between it and the magnetic disk recording surface by providing a heater in the head, basically, the protrusion value is adjusted so that reproduction can be carried out without causing medium contact by utilizing the fact that the clearance is changed by expansion/protrusion of the head flying surface along with power distribution to and heating of the heater; however, since a heater control value for a DA converter which sets the power supplied to the heater has been fixedly determined in a designing step, there has been a problem that highly precise clearance control that controls the clearance upon recording and reproduction to a constant target clearance in consideration of variation in the clearance that is different in each head cannot be performed. Moreover, there is a slight time lag after the clearance is changed by the protrusion caused by thermal expansion after power is distributed to the heater until it is stabilized; therefore, in the conventional clearance control by heater power distribution upon recording, it is in the state that the thermal expansion of the head is not completely saturated at a write starting part, and the clearance of the head does not reaches a target clearance; thus, there is a problem that the writing ability of the recording element is low, and the recording/reproduction characteristic is deteriorated at the write starting part. In order to solve these problems, when write is performed after preheating in which power distribution to the heater is started from a position before a target track is carried out, the thermal expansion of the head is completely saturated at the write starting position, it is stabilized at the target clearance, and the problem that the characteristic is deteriorated at the write starting part can be solved. However, even when the preheating is carried out, the head is thermally expanded when a recording current is caused to flow through the recording element upon recording; and, also in this case, there is a slight time lag until it is stabilized after the clearance is changed by protrusion which is caused by thermal expansion after flow of the recording current is started, and a phenomenon in which the clearance of the head is changed between the write starting sector and write sectors after that wherein the characteristic is changed occurs, which leads to inhibition of recording density improvement.